Fig. 2.

Effects of ${\mathrm{H}}_2$O incorporation on the Ct phase. (A) The axial ratios of Stv (squares) and Ct (circles) measured during decompression. The greater and smaller values for Ct are $c/a$ and $c/b$, respectively. The numbers are the weight percentage of ${\mathrm{H}}_2$O contained in dense silica polymorphs: 0% from ref. 20 and 3.2% from ref. 10. (B) Relationship between unit-cell volume expansion $\left(V/V_d\right)$ ($V_d$ is the unit-cell volume of dStv) and ${\mathrm{H}}_2$O content constructed from TGA and IR measurements (9, 21) (black circles). The gray area indicates $2\sigma$ uncertainties in the line fit of the data. We also include our DFT calculations (red symbols) for comparison. Inset shows the IR-active OH vibrational modes detected from a sample recovered from 45 GPa and 1740 K (the C–H peaks from diamond anvils and IR optics were masked with the gray rectangle). Some of the peak fitting results for A are presented in SI Appendix, Table S4. LDA, local density approximation. GGA, generalized gradient approximation.